Tracking the Sedge Wren

Mark Robbins, an ornithologist and collection manager at the KU Biodiversity Institute, investigates how sedge wrens navigate through the plains on their nomadic routes – where do they come from? What course do they take?

Although ornithologists have many tools to use to answer such questions, the sedge wren is a strange case. Bird tags are unreliable; the wrens’ travel patterns are too sporadic. Radio transmitters would weigh the wrens down – the birds only weigh about 9 grams, about the same as 6 paperclips.

Eventually, Robbins and his colleague Keith Hobson, a research scientist from Environment Canada, settled on the use of a chemical marker, a molecular tag found in the flesh and claws of the wrens.

The isotope deuterium, known as heavy hydrogen, falls intermingled in the rains of Kansas and the surrounding area. Insects of the area absorb some of this hydrogen in the water they drink. The wrens eat the insects and the deuterium becomes part of the wrens’ tissues.

Robbins and Hobson deduced that one could track where the wrens go by looking for traces of deuterium in the livers, muscles and claws of the birds. With this information they could track the wrens that had visited the breeding grounds of Kansas and the surrounding states. Different concentrations of deuterium would tell them where the birds came from.

But something strange sifted to the surface. The sedge wren has an extremely high metabolism and a heart rate seven times faster than a human’s. This rate of metabolism replaces the wren's cells much more quickly than in other animals.

Because of the extremely fast rate of cell turnover, Robbins and Hobson discovered that the deuterium levels in the wrens depleted too quickly to know where the birds had been. This is one of the first times ornithologists have run into this limitation, so the information gained will be valuable to future work.

Robbins hopes that radio transmitters will soon become small enough to put on something as small and spry as a sedge wren. In the meantime, the patterns of flight and dispersal for these tiny birds remain a mystery.